The embodiments of the invention generally relate to transistor manufacturing and more particularly to a method of removing sidewall spacers after silicide has been formed.
A gate sidewall spacer is generally used to space the source/drain diffusions and silicide away from the device channel region (the shallow extension implant bridges the gap). Once silicide is formed, the spacer is typically left in place where it may adversely affect subsequent processes. For example, the spacer crowds the limited area between closely spaced gates leaving inadequate room to form good CA contacts. Also, the presence of the spacer forces the stress liner further from the device channel thereby limiting the effectiveness of stress transfer to the channel.
The problem with simply stripping the nitride spacer after silicide formation is that etchants suitable for nitride removal also erode silicide. Use of a sacrificial nitride deposited preferentially on the silicide has been attempted but the beneficial effect of protecting the silicide tends to be overwhelmed by the need to etch longer to remove both the spacer and the additional nitride from the gate sidewall. The net result is the same or more silicide erosion than the case where no sacrificial nitride is used.
In view of the foregoing, an embodiment of the invention provides a method of forming an integrated circuit transistor. The method forms a gate conductor over a substrate, forms spacers (e.g., nitride spacers) on sides of the gate conductor, and implants an impurity into exposed regions of the substrate not protected by the gate conductor and the spacers. Then, the method forms a silicide on surfaces of the exposed regions of the substrate.
The method forms a conformal protective layer (e.g., an oxide or other similar material) over the silicide, the spacers, and the gate conductor. Next, the method forms a non-conformal sacrificial layer (e.g., nitride or other material that can be selectively removed with respect to the protective layer) over the protective layer. Because it is non-conformal, the sacrificial layer comprises relatively thinner regions that are over the spacers (e.g., the non-horizontal surfaces) and relatively thicker regions that are over the substrate and the top of the gate conductor (e.g., the horizontal surfaces).
The non-conformal nature of the sacrificial layer allows a subsequent partial etching process to partially etch the sacrificial layer. For example, the partial etching process could be continued only for a limited time such that the relatively thinner regions of the sacrificial layer that are over the spacers are completely removed and the relatively thicker regions of the sacrificial layer that are over the substrate are not removed (while the thicker regions may be thinned somewhat). The partial etching of the sacrificial layer only exposes portions of the protective layer that cover the spacers. Because the portions of the protective layer that cover the spacers are now exposed, the next step in the method can remove only those portions of the protective layer that cover the spacers, without removing the portions of the protective layer that cover the silicide (which, at this stage of processing, are still covered by the remnants of the thicker portions of the sacrificial layer). The removing of the portions of the protective layer that cover the spacers comprises a selective etching process that attacks protective material and does not attack sacrificial material. As the spacers are now exposed and the silicide is protected by the protective and sacrificial layers, the method can safely remove the spacers without affecting the silicide. The removing of the spacers comprises a selective etching process that attacks spacer material and does not attack protective material. This process that removes the spacers can simultaneously remove the remnants of the sacrificial layer if they are formed of the same material. The protective layer can remain in place or can be replaced by an insulator layer.
These and other aspects of the embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments of the invention and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments of the invention without departing from the spirit thereof, and the embodiments of the invention include all such modifications.